As is known, a sink unit comprises at least one fluid dispenser, usually in the form of a tap for dispensing water, for dispensing fluid into a receptacle, and a drain for draining the dispensed fluid from the receptacle. The sink unit may be provided with a single tap for dispensing water over a range of different temperatures, or two taps each of dispensing water of a respective temperature. The fluid dispenser may be located adjacent to the receptacle, mounted on the receptacle, or located on a wall.
The amount of energy used per year to heat water supplied to a sink unit can be significant. By way of example, a sink unit located in a public washroom with a medium footfall may be subject to around 200 uses per day. If each user of the sink unit chooses to wash with relatively hot water, typically dispensed at around 40° C., and to use around 330 ml of water per wash (calculated from a dispensed flow rate of 2 liters per minute and a wash time of 10 seconds) then the energy required to heat the water supplied to the sink unit over the course of the year from a temperature of around 18° C. to 40° C., may be around 625 kWh.
The temperature drop in the dispensed hot water as the dispensed water passes through the air and then over the hands of the user and on to the external surface of the receptacle to the drain is generally around 5° C. Consequently, the energy wasted as the hot water is expelled through the drain of the sink unit may be around 485 kWh per year.
It is known to provide a system for transferring heat from waste hot water expelled from the drain of a receptacle to incoming cold water to be supplied to the receptacle, and so reduce the amount of energy required to heat the incoming cold water before it is dispensed. For example, U.S. Pat. No. 4,291,423 describes a heat recovery system in which the waste hot water is conveyed from a receptacle to a channel located beneath the receptacle and housing a tube containing the incoming cold water. Heat is transferred from the waste hot water to the incoming cold water through the body of the tube.
However, the build up of soap, scum and hard water scale on the outer surface of the tube can impair the transfer of heat to the incoming cold water, and so the receptacle needs to be removable to enable a user to access the heat recovery system for cleaning. This can be inconvenient for the user.